183194-54-9Relevant academic research and scientific papers
Synthesis and glass transition behavior of high molecular weight styrene/4-acetoxystyene and styrene/4-hydroxystyrene gradient copolymers made via nitroxide-mediated controlled radical polymerization
Gray, Maisha K.,Zhou, Hongying,Nguyen, SonBinh T.,Torkelson, John M.
, p. 5586 - 5595 (2004)
Styrene (S)/4-acetoxystyrene (AS) gradient copolymers were synthesized by moderate-temperature, nitroxide-mediated controlled radical polymerization. Hydrolysis of styrene/4-acetoxystyrene gradient copolymers produced styrene/4-hydroxystyrene (HS) gradient copolymers. Molecular weight (MW) characterization via gel permeation chromatography demonstrated that these materials were made in a controlled manner, while intrinsic viscosity measurements revealed that the MWs exceeded 100 000 g/mol, with apparent viscosity-average MW values ranging between 100 000 and 385 000 g/mol, making these materials the first high MW gradient copolymers ever synthesized. Characterization of the glass transition temperature, Tg, revealed different behavior depending on the type of gradient copolymer produced. Using a normal thermal history for measuring Tg by differential scanning calorimetry (DSC), linear gradient copolymers exhibited one Tg, with a value intermediate to the Tgs of polystyrene (PS) and poly(4-acetoxystyrene) (PAS) or poly(4-hydroxystyrene) (PHS). In contrast, blocky gradient copolymers with overall S content ≥55 mol % yielded two Tgs, one near the Tg of PS and the other intermediate to the Tgs of PS and PAS or PHS, indicating microphase separation. When the cumulative composition of the blocky gradient copolymer was majority AS or HS, only one Tg was resolved, with a value near the Tg of PAS or PHS. For S/AS and S/HS gradient copolymers of identical chain length, overall fractional S content, and strength of gradient, physical aging at 90°C provided resolution of a second T g via DSC, allowing comment on how manifestations of microphase separation depend on gradient structure and the strength of the comonomer repulsive interactions.
Limitations in the synthesis of high molecular weight polymers via nitroxide-mediated controlled radical polymerization: Experimental studies
Gray, Maisha K.,Zhou, Hongying,Nguyen, SonBinh T.,Torkelson, John M.
, p. 5792 - 5797 (2003)
Limitations associated with preparing high molecular weight polystyrene (PS) by nitroxide-mediated controlled radical polymerization have been tested by considering the role of unimolecular initiator concentration on active polymer radical concentration and thus degree of polymerization. Recent theories ignoring autopolymerization effects lead to the conclusion that, at low monomer conversion, the number-average molecular weight, Mn, scales with the -2/3 power of unimolecular initiator concentration. Bulk polymerizations were done using either α-methylstyryl di-tert-butyl nitroxide (A-T) as unimolecular initiator or PS macroinitiator made from A-T. These initiators allow relatively low reaction temperature (77, 87, or 97 °C) and moderate, but not eliminate, the contribution of autopolymerization or thermal initiation of polymerization. By varying unimolecular initiator concentration over nearly 4 orders of magnitude, well-controlled PS, with polydispersity index ≤ 1.4, can be made with Mn values in the range 114 000-238 000 g/mol using either A-T as initiator or a PS macroinitiator. For conditions yielding controlled PS, in general the experimental Mn-initiator concentration data afforded good agreement with the -2/3 power-law expression and allowed estimation of the equilibrium constant for the capping-uncapping reaction. However, attempts to make controlled, higher molecular weight PS by further reducing initiator concentration resulted in loss of control due to autopolymerization effects. The impact of autopolymerization in producing well-controlled PS was evident from studies yielding a nearly constant conversion as a function of macroinitiator concentration.
